As others have said, the 'b' stands for 'baseband'. However the existing explanations aren't clear so I'll add some info. There are two ways to transmit information over wires: baseband and broadband. An example of broadband is (analog) cable TV: all the channels are combined together on one wire at once. The receiver selects different parts of the transmission spectrum to select different channels. You could receive every channel at once if your receiver were designed that way.

With baseband, only one signal is sent over the wire. That's simpler for digital transmission because the receiver doesn't have to do all the work of selecting 'channels' - there's just one. That channel contains the digital information (signal modulations) that are processed by the receiver into a data stream.

Obviously that's a greatly simplified answer, but that is the gist of baseband vs. broadband.

Funny aside: I had a Customer who had an old, creaky 10BROAD36 network around their campus (running on a campus-wide cable TV system). Quite the odd thing, it... en.wikipedia.org/wiki/10BROAD36
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Evan AndersonJan 14 '11 at 1:19